The present invention relates to illumination systems and, in particular, to an illuminator for light microscopes which provides center masking (masking the central area of the condenser) apparatus. The terms "mask" and "masking" as used herein refer to any method or device which has the effect of permitting or directing illumination light onto some areas of a condenser lens and preventing other areas of the condenser lens from receiving all or some of the available illumination light. When using the term "condenser lens," it will be understood that, in the context of a reflection microscope in which the present invention is operable, the condenser lens and the objective lens refer to the same focusing lens.
The idea of improving resolution and contrast in a light microscope by preventing areas of a condenser lens from receiving full illumination (center masking) has been discussed for many years, but has never been successfully incorporated into a commercial microscope. The two main reasons for the lack of commercialization of center masking are the absence of a practical device for producing such center masking and the lack of physical space at the rear aperture of a condenser of a high power microscope in which to locate center masking apparatus.
In order for center masking to be effective, it is necessary that the size of the area of the condenser masked be carefully matched to both the objective lens being used and the specimen under investigation. Too much or too little center masking can have a noticeable effect and, in some cases, even degrade the image relative to conventional axial illumination. Thus, to be fully effective, a center masking illumination system should provide continuous masking over a significant range, from very little masking (preferably zero) to a significant amount of masking, and be useful with a wide variety of commonly used condensers including high power condensers, as well as condensers with physically inaccessible rear apertures.
Mechanical devices are known which can provide center masking by expanding or contracting an opaque mask disposed at the center of the rear aperture of a condenser. These devices, which have been designed to occlude all illumination on a path directly to the objective to provide dark-field illumination, at their minimum possible occlusion, still provide too much center occlusion which, in many cases, can degrade the image rather than produce image enhancement. These prior art devices are also incompatible with condensers with either too little or no space available at the rear aperture to accommodate the masking mechanism.